1. Field of the Invention
The present invention relates to the repair of turbine blades and is particularly concerned with repair of turbine blades in which the turbine blade assembly has a plurality of blades interconnected by means of lacing wires, the lacing wires being wires or rods or other members that interconnect a plurality of blades by passing through holes in the blades, the purpose of the lacing wires being to prevent excessive relative movement between the blades and to generally dampen any vibration during operation of the turbine.
As aforementioned, the lacing wires may be wires, rods, tubes or other members and will hereinafter be referred to for the sake of convenience as lacing wires.
In use of turbines the blades are subjected to wear from contact with steam or particles carried by the steam which leads to erosion, particularly on the leading edge of the blade.
In view of the extremely high forces to which the blades are subjected, other damage may occur in the form of cracks which may necessitate complete removal and replacement of the turbine blade, or where the damage is not too severe, the damage can sometimes be rectified by repair to the blade.
2. Prior Art
British patent specification No. 2,091,139 discloses a method of repairing a turbine blade, the method including the step of filling a lacing wire hole with weld material prior to carrying out any welding or brazing operation in the vicinity of the lacing wire hole. When the blade was subsequently subjected to a stress relieving heat treatment, the discontinuity afforded by the lacing wire holes had been eliminated, thereby preventing in-built stress being caused by the heat treatment process around the discontinuity of the lacing wire hole.
Such a method of repair has been found highly satisfactory. However, problems occur where it has been found through improved techniques and equipment to enable repair of a turbine blade to be carried out while the blade is still in position on the rotor, since in view of the proximity of adjacent blades, removal of the lacing wire from the lacing wire hole to enable the hole to be filled has been found to be impossible unless the lacing wire is cut out completely and then it is not possible, without removing the blades from the rotor, to insert new lacing wire.
In practice, the lacing wire is made up of a plurality of pieces of material and in some cases the ends of the lacing wire pieces may be formed so as to overlap each other so as to permit of some relative movement between the lacing wire pieces. In other examples the lacing wire may be secured, for example by a tack weld or braze material, to each blade through which it passes and a small gap may be left between the ends of adjacent lacing wire so that the turbine blades are effectively laced in groups.
In some cases it will not be necessary to fill the lacing wire hole. However, it may still be necessary to remove the lacing wire from the blade to enable a satisfactory repair to be carried out since, for example, access to the blade with the lacing wire in position may not be possible.